1. Field of the Invention
The present invention relates to an ink jet recording material and a method of producing the same. Particularly, the present invention relates to an ink jet recording material capable of recording colored images having a high color density, a satisfactory gloss and an enhanced water resistance.
2. Description of the Related Art
An ink jet recording system is used to record colored images on a recording sheet by jetting imagewise ink drops through nozzles at a high speed to cohere the ink drops on a surface of the recording sheet and is advantageous in that full color printing is easy and in that the printing noise is low. This type of recording system use inks containing large amounts of a solvent and thus, to obtain a high color density of recorded images, the inks must be used in large amounts. Also, because the ink drops are continuously jetted, a disadvantage may occur in that, before early jetted ink drops are fully absorbed in the recording sheet to form early ink dots, later jetted ink drops reach the recording sheet to form later ink drops, and thus the later ink dots are fuse-connected to the early ink dots. Accordingly, the recording sheet for the ink jet recording system must be able to form ink dots with a high color density and a clear color tone, to absorb the jetted ink drops at a high absorbing rate so that even when the jetted ink drops overlap on the recording material surface, the ink drops are not mutually blotted.
Currently, in response to the rapid popularization of ink jet recording system, in the field of printing of publications and wrapping sheets, it is required to provide prints having a high gloss and a high color density. Especially, for full-color recording, plastic film-type and coated paper-type recording sheets are in high demand, because they exhibit excellent ink-receiving properties, for example, a high ink absorbing and fixing rate and a high ink absorption capacity, and thus can provide ink dots having a satisfactory form (truly circular) and a high sharpness.
Generally, since conventional inks for the ink jet recording system are water-soluble, the resultant ink images are disadvantageous in that the water and moisture resistance of the ink images are unsatisfactory. Accordingly, for the purpose of improving the moisture- and water-resistances, usually, a cationic resin is contained in the substrate paper sheet or the ink-receiving layer (recording layer).
For example, Japanese Examined Patent Publication No. 2-035,673 discloses an ink jet recording paper sheet prepared from a pulp slurry added with a pigment and a cationic resin. The pigment and the cationic resin contribute to enhancing the fixing of anionic dyes contained in the ink in the recording paper sheet, and to improving the moisture- and water-resistances of the fixed ink images.
Also, Japanese Unexamined Patent Publication No. 9-099,633 discloses a coated paper sheet usable for ink jet recording system, in which a coating layer comprising a pigment, for example, silica or alumina, is formed on a substrate sheet, to enhance the quality of images, for example, the sharpness of dots and the color density of images.
In the conventional coated paper sheets for the ink jet recording system, the coating layer comprises, as main components, pigment particles having a particle size in an order of several micrometers, a cationic resin and a binder, the pigment particles serve to absorb the ink and the cationic resin serves to fix the dyes contained in the absorbed ink.
The conventional ink jet recording material is, however, disadvantageous in that, since the pigment particles contained in the coating layer have a large particle size, the resultant coating layer is opaque, the surface thereof is rough, and the resultant ink images received in the coating layer have unsatisfactory gloss and color density.
An attempt has been made by the inventors of the present invention to enhance the color density of the recorded ink images. In this attempt, pigment colloidal particles having a particle size of 500 nm or less were prepared by pulverizing pigment particles, for example, silica particles, having a particle size in the order of micrometer by a mechanical dispersing method, and it was discovered that the resultant pigment colloidal particles contribute to enhancing the gloss and color density of the recorded ink images. However, the silica colloidal particles are anionic and thus have no fixing facility for the anionic dyes in the ink and the resultant recorded ink images exhibit an unsatisfactory moisture- and water-resistance. Also, it was discovered that when added with a cationic resin, the anionic pigment particles may be agglomerated and thus the transparency and surface smoothness of the coating layer may be decreased. Also, the agglomeration of the pigment particles due to the addition of the cationic resin may cause the viscosity of the coating liquid to increase, and thus the coating procedure with the viscosity-increased coating liquid may be difficult.
Also, in a previous attempt of the inventors of the present invention, a coating liquid for the ink jet recording material was prepared by absorbing a water-soluble resin into the surfaces of anionic colloidal particles, and then mixing the surface-treated colloidal particles with a cationic resin, as disclosed in Japanese Unexamined Patent Publication No. 9-263,039. In this method, the colloidal particles must be primary particles of the pigment which have a small surface area and have surfaces which can be fully covered by the water-soluble resin. If the colloidal particles are in the form of secondary particles which have a large ink absorption capacity, the particles have a significantly increased specific surface area, and thus it becomes difficult to completely cover the surfaces of the particles with the water-soluble resin. Also, the addition of the cationic resin may cause the particles of the pigment to be agglomerated and thus the viscosity of the resultant coating liquid to be increased. This phenomenon will cause the transparency and the surface smoothness of the resultant coating layer to be decreased.
If the water-soluble resin is used in an increased amount, the spaces formed between the colloidal particles and utilized to absorb the ink are decreased and thus the ink-absorption rate and capacity of the coating layer (ink-receiving layer) are decreased.
Generally speaking, in the preparation of an ink receiving layer, the smaller the particle size of the pigment particles contained in the ink receiving layer, the higher the transparency, surface smoothness and surface gloss of the resultant ink receiving layer and the color density of the recorded ink images. The pulverization of pigment particles is carried out basically by applying three types of forces, namely, a shearing force, an impact force and a compression force, alone or in combination, to the pigment particles. In a pulverization procedure in which the shearing force is mainly utilized, a conventional mixer and a Cowles disperser are used. In a pulverization procedure in which the impacting force is mainly utilized, a conventional jet mill is used. Also, in a pulverization procedure in which a combination of the shearing force with the impacting force is utilized, a conventional sand mill, a ball mill or a roll mill can be used.
In a conventional method for dispersing pigment particles for paints, usually a mechanical agitation-dispersing method using a mixer or Cowless disperser is used. This conventional method is, however, unsatisfactory to pulverize the pigment particles so as to cause the particle size of the pigment particles to be decreased, and to divide the agglomerates of secondary particles which have been formed from agglomerates of primary particles having a poor dispersion-stability into secondary particles having a size smaller than that of the secondary particle agglomerates.
In comparison with a mixer, a sand mill and a ball mill are excellent in dispersing facility and pulverizing facility. These mills utilize balls or beads as a dispersing medium and thus they are referred to as a dispersing medium-type disperser. When this type of disperser is used for the preparation of a coating liquid having a high viscosity, the shearing force is cut by the cushioning phenomenon of the dispersing medium. Therefore, the dispersing medium-type disperser is usable only for coating liquids having a low or medium degree of viscosity. For dispersing a paint having a high viscosity, the roll mill is advantageously used. However, the roll mill is unsatisfactory in its dispersing effect.
Japanese Unexamined Patent Publication No. 5-32413 discloses a method of pulverizing alumina sol secondary particles in which primary particles are easily agglomerated with each other, by using a ultrasonic vibration disperser in which not only a high shearing force but also a cavitation mechanism are utilized. However, the resultant dispersed alumina particles are unsatisfactory in that the particle sizes of the resultant secondary particles are too large and the resultant particle size distribution is too wide. Therefore, when the resultant finely dispersed paint is used, the resultant ink receiving layer is unsatisfactory due to the low transparency thereof. Also, since the particle size distribution is too wide, the adhesion of the particles to each other through a binder is insufficient and thus the resultant ink receiving layer may be easily cracked. Also, the dispersion procedure of the pigment particles needs a long time and a large amount of labor and thus the efficiency of the coating liquid preparation procedure for the ink receiving layer is poor.